The present invention pertains to the electrical art and, more particularly, to an improvement in a voltage controlled oscillator.
Voltage variable capacitor controlled oscillators are well-known, particularly in the communication art. There, it has been known to apply a low frequency information signal, such as a signal transduced from speech information, and apply that signal to a voltage variable capacitor which is in tuning configuration in an oscillator. The change in capacitance of the voltage variable capacitor results in a deviation in the tuning of the oscillator stage whereby frequency modulation is effected.
It is generally desirable to be able to change the center freuqency of oscillation of the oscillator without affecting the linearity of the frequency modulation. That is, it is desirable that a linear change in input signal result in a linear deviation in the frequency modulation produced signal. However, whereas in a given frequency this can be assured, once the carrier of the frequency modulated signal is changed, the prior art has known only two ways to readjust the system to achieve the desired linearity. One prior art approach to linearization has been to change the DC voltage applied to the voltage variable capacity diodes. By suitably changing the DC voltage applied to the diodes, the net reactance from the diodes may be located in a range such that the applied information signal produces linear frequency modulation. An alternate prior art approach has been to couple the diodes to the oscillator circuitry through a transmission line. By changing the length of the transmission line, a corresponding change in the effect of the capacitance of the voltage variable capacitors, as seen by the oscillating circuitry, is affected, thereby insuring linear frequency modulation.
These prior art approaches to linearizing the frequency modulation have proved unsatisfactory. For the approach wherein the DC voltage to the voltage variable capacity diodes is varied, special electrical circuitry has to be provided which assures that the DC voltage applied to the voltage variable capacity diodes precisely locates their impedance at the point of linearization. This circuitry has proved difficult to fabricate.
As for the prior art approach of varying the length of a transmission line, this requires a mechanical transmission line structure which may be easily, yet precisely varied. Such a structure is both difficult and expensive to manufacture.